COURTESY OF TIMOTHY TRICAS
Butterfly fish were placed into a glass jar and set into the territory of the same species of territorial, coral-feeding fish in an experiment off the Kona Coast. The tactic, known as the "Model Bottle" experiment, helped University of Hawaii researchers record sounds from the fish.

University of Hawaii animal behaviorist Timothy Tricas put a butterfly fish in a bottle, and his scientific wish came true with a grunt.

Tricas, who has long been intrigued by the fish's different colors, feeding habits and diverse social behaviors, recently discovered that more than half of its 126 species might be able to discern and make sounds.

The secret lies in a unique arrangement that appears to connect the fish's swim bladder with its water-motion-detecting lateral lines. Previous analysis suggested the setup could be a system to enhance hearing, but that theory had never been tested.

Until Tricas figured that if the fish had a hearing system, it must also be able to make sounds. In a test off the Kona Coast known as the "Model Bottle" experiment, he placed a pair of butterfly fish into a glass jar and lowered it into the territory of the same species of aggressive, coral-feeding fish. Within seconds, waterproof video and sound recorders detected a soft grunt by the bottled fish and a click created by the tail slaps of the resident fish at Puako Reef.

"They are intimidated, and they know it's not their territory. It could be a warning call, or an agonistic sound to their mates," explained Tricas, a zoology professor at UH-Manoa. He suspects the grunt originates from the grinding of teeth at the back of the fish's throat, but a UH graduate student is trying to confirm the sound's source.

Meanwhile, the tail slap is probably used to "push water at the enemy and create a loud click," he said.

For the past three years, Tricas and his students have spent hundreds of hours underwater and also inside his lab at the Hawaii Institute of Marine Biology to study the butterfly fish with a grant from the National Science Foundation.

In lab tests, they used brain wave recording techniques to find evidence that the link between the fish's lateral line and its swim bladder -- present in 86 of the species -- is also a mechanism that boosts sound detection by the fish's ear. By pumping minuscule amounts of Vaseline into two hornlike features attached to the swim bladder, a fish becomes less sensitive to sound, Tricas said.

"After those little injections to the horn, the fishes actually swam closer together. Those that did not receive the injection in the horn did not change the pairing," Tricas said. "You block that special connection, basically, and the fish's hearing decreases."

Tricas has been fascinated with butterfly fish for almost three decades, mostly because of his curiosity about the species' different social behaviors. Some of the fish remain faithful to schools, others are monogamous and might stay for life with a single mate, while a small minority spend their entire life mostly alone on coral plates.

And Tricas believes the sounds he discovered could help researchers understand the muscles and structures that the butterfly fish use to make soft sounds. Unlike the offensive click, the grunt produced by the trapped butterfly fish pair and other discovered sounds are as weak as a whisper. That, Tricas said, could be one reason why many of these fish are highly social and, to hear each other, swim less than a body length apart.

"We've never heard them before, and the reason is because the sound is very weak," he said. "Perhaps one of the reasons that that pairing behavior has evolved, in this group, is because it facilitates hearing."

The findings suggest that, while quiet to the human ear, oceans might be much noisier than they appear. Of the world's 25,000 fish species, between 800 to 1,000 are known to make sounds, though only 2,000 or so have been investigated for sound production, Tricas said.

"We've only scratched the surface of knowledge about the types of fish that make sound and, most importantly, why they produce the sounds," he said.